136/475Khz ATU

I built this ATU originally to use with my FT817ND when I was experimenting with QRSS grabbing on the band. It can cover 136Khz - 600Khz adequately as it stands. I also added an Led-O-Meter to it for my 1w low powered QRSS experiments when using my QRP Labs U3 transmitter.

The inductance is wound on a ferrite rod of 9mm diameter and a Mu of about 2000. Each 100 turns gave me about 500mH. For receive purposes simply winding to the number of turns was adequate, but for transmit purposes you'll need to optimise those turns.  Hint - on an FT817 you'll find that switching the pre-amp or IPO to the OFF position will increase receiver sensitivity.  The pre-amp was never really designed to be used down there, and so it tends to at as an attenuator. A cheap polyvaricon (as often used in portable AM broadcast radios can be ganged up to make an approx 600pF variable cap.

I found that having as little as about 6 metres of wire fed against earth gave superb receive performance on the FT817. I even used it on transmit in the attic ! Yes folks, I was using WSPR and QRSS signals on 475Khz using a loft antenna ! Best DX was to Norway. I estimated that my effective radiated power was 0.25mW. One quarter of one milliwatt.  It was in the middle of winter and I think it was LA3EQ that received me. 

Warning, don't expect it to be easy to radiate a good signal on the 136/475Khz bands. If you think it's jut a dropping the next band down from 160m then you're gonna be in for a very big surprise. I couldn't even hear my own carrier 100 metres way if I used a portable receiver tuned to the band. And yet the digital signals like WSPR can be detected nearly 1000 miles away.

Note: This mini ATU is best really suited for receive purposes. Yes I have used it for transmit purposes but you have to excercise a little more care with how a good match is achieved. The switched inductance is a fairly blunt instrument for achieving a wide frequency range, but may not always achieve a 1:1.1 SWR. Maybe a mechanical movement of the ferrite rod within the coil would achieve perfection ?